Although I suspect the issue you are having is with permissions so you might want to add sudo in front of that command. You could also change the permissions on that subdirectory (careful now) and then from the GUI cut all content and paste it where you need it. But at this point you might as well use the command line directly.
You have to use the user account you set up for the web server when you configured it. You keep running into these issues because you are not using the web admin account you needed to configure when you set up the web server. Try searching the web how to configure that server software (apache, nginx, etc). You could also search how to configure admin account for it as well.
To be clear: I don't think you should change the permissions on this type of directories. Using sudo is just a few more keystrokes away, and the convenience does not outweight the security risk here.
I understand you are doing this locally for practicing, perhaps using a virtual machine or something, and so it's "harmless". But this really isn't the way to go. If you later want to put out a web server online (which is kinda the point of a web server...) and it gets hacked, it will most likely join a botnet which will then be used for who knows what.
I was going to give the instructions on how to change the permissions but decided against this, sorry. For one, because you can find this information very easily anyway, if you haven't already. At it least this way it won't come from me. The fact that you can't drag and drop files in and out of sensitive folders is a good thing and is it's by design.